1. Field of the Invention
The present invention relates to an exposure apparatus used in a photolithography process in the manufacture of, e.g., a semiconductor element, a liquid crystal display element, a thin-film magnetic head, or the like and, more particularly, to a scanning type exposure apparatus for transferring a pattern on a mask (or a reticle) onto a substrate by synchronously moving the mask and the substrate.
2. Related Background Art
In the photolithography process in the manufacture of semiconductor elements, a projection exposure apparatus for transferring a pattern formed on a mask or reticle (to be generally referred to as a "reticle" hereinafter) onto a substrate (wafer) coated with a photosensitive material (photoresist) via a projection optical system, in particular, a step-and-repeat type reduction production exposure apparatus (stepper), is popularly used. Recently, in association with an increase in size and a decrease in line width of semiconductor elements, it has been required to widen the image field of the projection optical system and to improve the resolution of the projection optical system. However, it is very difficult in terms of design and manufacture to realize both the high resolution and the wide field of the projection optical system. Thus, as disclosed in, e.g., U.S. Pat. Nos. 4,747,678, 4,924,257, 5,194,893, and 5,281,996, a scanning type exposure apparatus, which illuminates only a partial area having a predetermined shape (e.g., a rectangular shape, arcuate shape, hexagonal shape, rhombic shape, or the like) on a reticle with light, and exposes a pattern on the reticle onto a wafer by synchronously moving the reticle and wafer along a direction perpendicular to the optical axis of the -projection optical system, is receiving a lot of attention. In the scanning type exposure apparatus, even when the image field of the projection optical system is small, a large-area pattern image can be exposed onto the wafer, and the resolution of the projection optical system can be relatively easily improved.
FIG. 5A illustrates a conventional scanning type projection exposure apparatus. Referring to FIG. 5A, exposure light EL emerging from an illumination system IL illuminates an illumination area 32 on a reticle 12 at an even illuminance. A projection optical system 8 projects a pattern in the illumination area 32 on the reticle 12 onto a wafer 5. In scanning exposure, the reticle 12 is moved by a reticle stage RST at a speed V.sub.R in a -Y direction (left direction in the plane of the drawing) with respect to the illumination area 32. In synchronism with this movement, the wafer 5 is moved by a wafer stage WST at a speed V.sub.W (=.beta..times.V.sub.R, .beta.: the projection magnification of the projection optical system 8) in a +Y direction (right direction in the plane of the drawing) with respect to a projection area (exposure area similar to the illumination area 32) 32W defined by the projection optical system 8. With these movements, a shot area SA on the wafer 5 is scanned in the Y direction with respect to the exposure area 32W, as shown in FIG. 5B, and the pattern image on the reticle 12 is scanning-exposed on the shot area SA.
FIG. 6 is a functional block diagram showing a control system of the scanning type exposure apparatus shown in FIG. 5A. Referring to FIG. 6, when a speed command signal indicating a scanning speed is input to a speed control system 61 for the wafer stage, the speed control system 61 drives the wafer stage WST in the Y direction, and performs speed control, so that the moving speed V.sub.W of the wafer stage WST coincides with the speed command. Normally, the position of the wafer stage WST is measured by a laser interferometer. However, in FIG. 6, a multiplier 66 multiplies a speed signal (a signal indicating the speed V.sub.W) output from the speed control system 61 by 1/.beta.. Then, this speed signal from the multiplier 66 is supplied to an integrator 62, and the output signal from the integrator 62 is used as a position signal indicating a position Y.sub.W, in the Y direction, of the wafer stage WST.
On the other hand, a speed signal (a signal indicating the speed V.sub.R) output from a speed control system 64 for the reticle stage is supplied to an integrator 65, and the output signal from the integrator 65 is used as a position signal indicating a position Y.sub.R, in the Y direction, of the reticle stage RST. The position signals from the integrators 62 and 65 are input to a subtracter 63, and a signal indicating a positional difference (Y.sub.W -Y.sub.R) output from the subtracter 63 is supplied to the speed control system 64. For the sake of simplicity, the projection magnification .beta. of the projection optical system 8 is set to be 1.
When the wafer stage WST begins to move by the speed control system 61 to follow the speed command signal, the signal which indicates the difference between the position Y.sub.R of the reticle stage RST and the position Y.sub.W of the wafer stage WST (the signal output from the subtracter 63) changes, and is supplied to the speed control system 64 to accelerate the reticle 12 in a direction indicated by the difference. The speed control system 64 comprises a PID controller (proportional, integral, derivative controller) having an integral function, and the like, and performs acceleration control of the reticle stage RST until the above-mentioned difference (Y.sub.W -Y.sub.R) becomes zero. Thus, the reticle 12 and the wafer 5 are synchronously scanned.
In the above-mentioned prior art (FIG. 6), the speed command signal is supplied to the speed control system 61 for the wafer stage, and the signal indicating the difference between the positions Y.sub.W and Y.sub.R of the wafer and reticle stages is supplied to the speed control system 64 for the reticle stage. Thus, after the movement of the wafer stage WST is detected, the scanning speed of the reticle stage RST is increased/decreased. For this reason, a time from acceleration of the reticle and wafer to the beginning of synchronous scanning is long.